1. Technical Field
This disclosure pertains to regulators for power supplies and, more particularly, to regulators for isolated flyback power supplies that use primary side sensing.
2. Description of Related Art
Power supplies that are used in telecommunications, health care, industry, and other applications may require electrical isolation between the input to the supply and the output from the supply. A transformer is often used to provide this isolation.
The transformer is often configured in what is known as a flyback converter. A flyback converter often regulates the output of the power supply by controlling a series of pulses that are delivered to the primary winding of the transformer. When the output needs to be increased, the on-time and/or the frequency of the pulses may be increased. Conversely, when the output needs to be decreased, the reverse may take place.
To determine how the pulses must be altered to achieve a desired output, the output of the supply is usually sensed. When complete electrical isolation must be maintained between the input and the output, this sensing must also be done in an electrically isolated manner.
To achieve this electrical isolation, an optoisolator is sometimes used to relay the output level back to the input control system. The feedback in this configuration utilizes an optical path over which electrons cannot travel. Optoisolators, however, can increase the size of the power supply, increase costs, and may limit the ability of the power supply to make needed adjustments quickly.
Another technique has been to monitor the voltage on the primary winding of the transformer in the flyback converter, commonly referred to as “primary side sensing.” This monitoring has typically been done during each off period of the pulses when the primary winding has been disconnected from the supply voltage and while current is still traveling through the secondary winding of the transformer. This techniques works due to inherent characteristics of transformer. These inherent characteristics cause the voltage on the primary winding during this period to be approximately equal to the output voltage of the power supply, times the ratio of turns in the primary and secondary windings.
The output regulation that is accomplished using primary side sensing, however, may not be adequate. The regulation may not be as fast and/or as accurate as is needed. Efforts to improve the regulation, moreover, may cause the effectiveness of the regulator to become dependent upon the specific characteristics of the transformer that is used with it. This may make it difficult to design a single regulator that can be effectively used with a broad variety of transformers.